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76 The PCB Design Magazine • July 2016 the file. To quote from the Gerber specification: The responsibilities are obvious and plain. Wri- ters must write valid and robust files and readers must process such files correctly. Writers are not responsible for navigating around problems in the readers, nor are readers responsible for sol- ving problems in the writers. It is therefore extremely important that you check that your files are valid. Invalid files can cause viewers to throw error messages like the one in Figure 1, taken from GC-Prevue: These messages clearly indicate that there is something very wrong with the file. The question is, what you do if you see such errors? It's not easy. Low resolution is often the root cause of problems, so it is worth trying to output the file at the resolution recommended in Chapter 10 in this series. The only safe solution is to fix the bugs in the Gerber output software. It is therefore essential that you provide detailed information of the problem to your software supplier so that the bug can be fixed for the future. That said, the chances are that your board cannot wait for this fix and you have no way to output a valid file. This is then a conundrum. You could send the invalid data with the necessary caveats and hope that your fabricator's software, like your reader, will reverse engineer the intended image correctly. If it does, all is well. But this is a risk, so if you decide to do this, always include a netlist as a safeguard, as advised in Chapter 8 in this series. You can also ask your fabricator to send you the images he generates in CAM, so that you can check them for errors. THE GERBER GUIDE, CHAPTERS 15 & 16 Figure 1: Error messages in a viewer.